1. Field of the Invention
The present invention relates to the use of hydrogen-air fuel cells in portable devices. Such fuel cells are generally formed of one or several silicon wafers, each of which contains a large number of microcells.
2. Discussion of the Related Art
Hydrogen-air fuel cells especially aim at equipping portable electronic equipment such as computers, telephones, music readers, and others.
FIG. 1 shows an example of an integrated microcell fuel cell formed by using microelectronics techniques. This cell is formed on a silicon wafer 1 coated with a first thin insulating layer 2 and with a second thicker insulating layer 3. An opening is formed in this portion of insulating layer 3. In this opening is housed a stack of a catalyst support 4, of a first catalyst layer 5, of an electrolyte 6, and of a second catalyst layer 7. This layer assembly forms the active stack of the fuel cell. An electrode 10 placed on first insulating layer 2 enables taking a contact on the lower surface of the fuel cell, on support 4. An opening in second insulating layer 3 enables accessing electrode 10. An upper electrode 11 enables taking a contact on upper catalyst layer 7. Electrodes 10 and 11 are perforated and channels 13 are formed in silicon wafer 1 opposite to the perforations in the lower surface metallization. Lower electrode 10 and upper electrode 12 respectively form an anode collector and a cathode collector.
Electrolyte 6 is, for example, a polymer acid such as Nafion in solid form and catalyst layers 5 and 7 are, for example, carbon and platinum based layers. This is an example of embodiment only. Various types of fuel cells that can be formed as illustrated in FIG. 1 are known in the art.
To operate the fuel cell, hydrogen is injected in the direction indicated by arrow H2 on the lower surface side and air (oxygen carrier) is injected on the upper surface side. The hydrogen is “broken down” at the level of catalyst layer 5 to form, on the one hand, H+ protons which direct towards electrolyte 6 and, on the other hand, electrons which direct towards the outside of the cell through anode collector 10. The H+ protons cross electrolyte 6 to join catalyst layer 7 where they recombine with oxygen and with electrons to form water microdroplets.
It should be underlined that FIG. 1 is not to scale. In particular, silicon wafer 1 typically has a thickness on the order of from 250 to 700 μm while the active stack of layers 4 to 7 typically has a thickness on the order of from 30 to 50 μm.
A conventional fuel cell is formed of a large number of adjacent cells of the type shown in FIG. 1, generally several hundreds, integrated on the same substrate and properly connected. In practice, a fuel cell battery (which will be called “cell” hereafter) usable in portable devices such as cell phones should have a surface area in contact with the air on the order of a few square centimeters (for example, with a side from 2 to 3 cm).
The Applicants have noted that, when such fuel cells are used in portable devices, and when the cell stopped being used for a given time, the restarting would be difficult, that is, for a given time after the restarting, the fuel cell would be incapable of providing the desired nominal current.